1. Field of the Invention
The invention relates to an auto-focus detection device and an auto-focus detection method in an auto-focus detection camera.
2. Description of Related Art
Auto-focus detection cameras have a focus detection area in the center or similar selected areas of the photographic field. The photo lens is automatically driven so that a subject caught in this focus detection area is focused. Thus, the photographer can place the principal subjecting focus by causing the desired subject (referred to hereafter as "the principal subject") to be positioned in the focus detection area within the photographic field.
A conventionally known focus detection method used by the auto-focus detection device of cameras is the phase difference detection method. In this method, two images in parallax with the subject are conducted onto a pair of image sensors, the relative shifting amount of the two images is calculated from the image output of the pair of image sensors, and the focus condition is determined. This method is described using FIG. 4.
After the light rays, incident through region 101 of the object lens 100, converge at the focal point for the film surface 600 or the focal surface, light rays are composed into an image on image sensor A through a band pass filter 700, a field of vision mask 200, a filter lens 300, a diaphragm aperture 401, and a recomposing lens 501. In the same manner, after the light rays incident through region 102 of the object lens 100 converge at the focal point for the film surface 600 or the focal surface, the light rays are composed into an image on image sensor B through the band pass filter 700, the field of vision mask 200, the filter lens 300, a diaphragm aperture 402, and a recomposing lens 502.
The pair of subject images, that have been composed on the image sensors A and B, will diverge in the so-called front focus condition, in which the object lens 100 forms a clear image of the subject in front of a pre-set focal surface, and will converge in the so-called back focus condition, in which a clear image of the subject is formed behind the pre-set focal surface. In the so-called focused condition, in which a clear image is formed exactly at the pre-set focal surface, the subject images on the image sensors A and B correspond to each other.
The pair of subject images undergoes photoelectric conversion by the image sensors A and B and is converted to electrical signals. By mathematically processing the signals and calculating the relative positions of the two subject images, the focus adjustment condition, which here refers to the amount and direction of separation from the focused condition (referred to hereafter as "the defocus amount"), of the object lens 100 is determined.
The focus detection area becomes the area in which the image sensors A and B are projected through the recomposing lenses 501 and 502 and reflects the overlap in the vicinity of the pre-set focal surface.
The mathematical processing method by which the defocus amount is calculated will now be described. The image sensors A and B each comprise a plurality of photoelectric converting elements and, as shown in FIGS. 5 (a) and (b), output a plurality of photoelectrically converted outputs a1 . . . an and b1 . . . bn, respectively. Correlation calculations are carried out while each data line is shifted by a specified data amount L. Specifically, the correlation amount C (L) is calculated through the following expression 1. ##EQU1##
L is an integer that corresponds to the data line shift amount as described above. The initial value k and the ultimate value r may be changed, depending on the shift amount L.
Among the correlation amounts C (L) that are obtained, the amount which consists of the shift amount that gives the correlation amount with the absolute minimum value and which is associated with a constant that is determined by the optical system shown in FIG. 4 and the pitch width of the photoelectric converting elements of the image sensors becomes the defocus amount. However, as shown in FIG. 5 (c), the correlation amounts C (L) are scattered values, and the smallest unit of the defocus amount that can be detected is limited by the pitch width of the photoelectric converting elements of the image sensors. Therefore a method is proposed in which a new absolute minimum value Cex is calculated by carrying out an interpolation calculation from the scattered correlation amounts C (L) and then carrying out detailed focus detection (See Japanese Unexamined Patent Application Sho 60-37513). This is a method which calculates Cex from the correlation amount C (0), which is the absolute minimum value, as shown in FIG. 6, and the correlation amounts C (1) and C (-1), which are the shift amounts on either side of the correlation amount C (0). The shift amount Fm that gives the absolute minimum value Cex and the defocus amount DF are calculated according to the following equations. EQU DF=Kf*Fm EQU Fm=L+DL/E EQU DL={C(-1)-C(1)}/2 EQU Cex=C(0)-.vertline.DL.vertline. EQU E=MAX [{C(-1)-C(0)}, {C(1)-C(0)}] (2)
MAX {Ca, Cb} means that the largest value among Ca and Cb is selected. Kf is a constant that is determined by the optical system shown in FIG. 4 and by the pitch width of the photoelectric converting elements of the image sensors.
It is necessary to determine whether the defocus amount thus obtained indicates the true defocus amount, or whether the defocus amount has undergone vibration of the correlation amount due to noise or the like. A defocus amount that satisfies the following expressions is considered to be reliable. EQU E&gt;E1 and Cex/E&lt;G1 (3)
where E1 and G1 are specified values. E is a value that depends on the contrast of the subject; the larger the value, the higher the contrast and the higher the reliability. Cex/E depends mainly on the degree of coincidence of the images; the closer to 0, the higher the reliability.
When it is determined that the defocus amount is reliable, the object lens 100 is driven to bring the subject into focus, based on the defocus amount DF.
CCD's or the like, consisting of charge accumulating-style photoelectric converting elements, are generally used for each of the element sensors that form the image sensors. If the size of the image output of these image sensors is not adequate, high precision cannot be obtained and detection becomes impossible.
For example, if a picture such as that shown in FIG. 3 (a) is viewed, it is desirable for an image output, such as that shown in FIG. 3 (c), be obtained. In this figure, Vsat indicates the saturation level of the photoelectric converting elements. However, if the accumulation time interval is short, a low contrast output results, as shown in FIG. 3 (b). Conversely, if the accumulation time interval is long, the original contrast disappears, as shown in FIG. 3 (d). Therefore it is necessary to obtain an image output of the appropriate size and the charge accumulation must be carried out over an appropriate accumulation time interval. Examples are given hereafter of methods for calculating this appropriate accumulation time interval.
One example of an accumulation time interval calculating method is a method in which the peak value or the like of the output at the next accumulation becomes the appropriate value, based on the accumulation time interval of the previous accumulation and on the image output of the image sensor. For example, if an output such as that in FIG. 3 (b) is obtained, the accumulation time interval at that time is Tb and the peak output is Vb. In this case, in order to obtain the appropriate output such as that shown in FIG. 3 (c) for the next accumulation operation, the accumulation time interval should be set to Tc=(Vc/Vb).times.Tb. In this formula, Vc is the target value; Vc=A.times.Vsat, where A is a positive real number less than 1. The size of A is established by the "appropriate size" of the image output. If A is too small, the contrast will be continually low; conversely, if A is too large, the image output will immediately saturate with merely a slight increase of the subject brightness. However, the size of the image output described above does not include noise components from dark currents or the like. This method is hereafter designated as soft AGC (Auto Gain Control).
Another example is a method in which an AGC sensor is provided near the image sensor, and an appropriate accumulation time interval is calculated based on the output of the AGC sensor. This method is hereafter designated as hard AGC.
In cases in which the subject is dark and an image output of the appropriate size is sought, the accumulation time interval becomes too long. Therefore the accumulation time interval is shortened, and the obtained image output is amplified through an amplifier (amp). If this is done, an appropriate image output can be obtained for both bright subjects and dark subjects by switching the amplifier gain according to the subject brightness.
However, the saturation level of accumulation-type photoelectric converting elements changes when the amplifier gain is switched. For example, the amplifier gain is taken to have an amplification ratio of one in the lowest case and two in the highest case. In addition, the accumulation capacity is taken to be 3.8 V, and the dynamic range of the circuit on the amp side is taken to be 4 V.
When the amplifier gain is lowest, the charge accumulates to the full accumulation capacity, and becomes saturated through the overflow of the capacity. In the case of the above example, since accumulation to 3.8 V takes place through the sensors and is multiplied by one through the amp, the saturation level is 3.8 V.
When the amplifier gain is high, the saturation level is set by the amp side circuit, and charge does not accumulate to the full capacity. Thus, since accumulation to 2 V takes place through the sensors and is multiplied by two to become the 4 V of the full dynamic range, the saturation level becomes 4 V.
Thus, when the accumulation time interval and the dynamic range of the amp side circuit are different, a problem arises in that the appropriate output and the appropriate accumulation time interval in the case of one amplifier gain are not necessarily appropriate in the case of a different amplifier gain.
Raising the accumulation capacity and the dynamic range results in an increase in the image resolution, but there is a simultaneous increase in dark current, and the layout of the sensor base plate also becomes a difficult problem.